Method of producing steel for steel pipe excellent in sour-resistance performance

ABSTRACT

The steel for steel pipes of the present invention is the one for steel pipes excellent in sour-resistance performance including C, Mn, Si, P, S, Ti, Al, Ca, N and O, and optionally including a predetermined amount of one or more of Cr, Ni, Cu, Mo, V, B and Nb, in which inclusions in the steel have Ca, Al, O and S as main components, the CaO content in the inclusions is 30 to 80%, the ratio of the N content in the steel (ppm) to the CaO content in the inclusions (%) is from 0.28 to 2.0, and the CaS content in the inclusions is 25% or less. In addition, the method of producing steel for steel pipes of the present invention is to produce steel for steel pipes in which Ca is added so that the ratio of the N content in the steel to the amount of Ca addition (kg/t) into the molten steel is from 200 to 857. According to the production method of the present invention, a slag composition, temperature-raising heating of molten steel, stirring treatment of molten steel and slag, and the Ca addition are optimized, whereby high-strength HIC resistant steel for steel pipes that exhibit excellent sour-resistance performance and cleanliness can be stably manufactured.

TECHNICAL FIELD

The present invention relates to a method of melting and refining anextra-low-sulfur high-cleanliness steel excellent in corrosionresistance, and particularly to a method of melting and refining steelfor high-strength steel pipes improved in sour-resistance performance bycontrolling a chemical composition of non-metallic inclusions in steel,specifically by decreasing the effect of carbonitrides.

BACKGROUND ART

Conventionally, hydrogen-induced cracking resistance (HIC resistance)and sulfide stress corrosion cracking resistance (SSCC resistance), andthe like have been required for materials for line pipes. Steelexcellent in these properties are called HIC resistant steel,sour-resistant steel, and the like.

Up to now, an inclusions-morphology control technology by Ca treatmenthas been developed to improve this HIC resistance performance. Theinitial object of Ca treatment was to inhibit HIC attributable to MnS bymorphing MnS as sulfide into Ca-type inclusions. However, it came tolight that HIC is attributed to Ca-type oxide and sulfide inclusions(oxysulfide inclusions) other than MnS, for example, inclusionsrepresented by Ca—Al—O—S, Ca—S, and Ca—S—O. And, the need for morphologycontrol of Ca-type oxysulfides in addition to MnS has been recognized.Thus, many technologies that attempt to control inclusions-morphologyhave been developed. For instance, Japanese Patent ApplicationPublication No. 56-98415, etc. discloses steel production methods thatdecrease the number of inclusions.

In addition, as the environment of pipes in use become hostile, furtherenhancement of sour-resistance performance and higher strength aredemanded and the development of inclusions-morphology control technologyis also conducted to satisfy the demand. Japanese Patent ApplicationPublication No. 06-330139 discloses a method of controlling inclusionsthat involves adding Ca, Al and Si so as to satisfy a specifiedrelational expression for steel types of X42 to 65 grades of APIStandards.

Meanwhile, in recent years, much higher sour-resistance performance andstrength in steel have been demanded and more advanced technologydevelopment has been pursued. Japanese Patent Application PublicationNo. 2005-60820 discloses a technology that improves sour-resistanceperformance by attempting the dispersion of carbonitrides for a steelgrade equal to or higher than the X65 grade of API Standards. Inaddition, Japanese Patent application Publication No. 2003-313638discloses steel obtained by dispersing and depositing precipitatesincluding Ti and W for a similar steel type which is equal to or higherthan the X65 grade of API Standards. Moreover, Japanese PatentApplication Publication No. 2001-11528 discloses a method for meltingand refining steels that controls the composition of Ca—Al—O—S-typeinclusions by adjusting the amount of Ca addition such that the Caconcentration satisfies a predetermined relation according to the S andO concentrations in molten steel.

Then, the present inventors found that bulky TiN-type inclusionsexceeding 30 μm in size become the initiation point of HIC and proposedsteel in which these are reduced and a method of controlling the size ofTiN to 10 to 30 μm by use of Ca—Al-type inclusions in WO2005/075694.

As described above, the morphology control technology for inclusions byCa treatment has been upgraded according to performance demand forsteel, and the technology has been developed from simple addition of Cato inhibiting CaS generation and improving cleanliness to controllingcomposition of Ca-type inclusions and further to the fine dispersion andprecipitation of carbonitride-type inclusions.

Incidentally, recently, higher sour-resistance performance and strengthhave been demanded as previously described. For these demands, followingproblems are present. A first problem is to address the instability ofsour-resistance performance. In other words, the technology intended forhigh-strength steel is for the dispersion of carbonitrides and thecomposition control of Ca-type inclusions. Although the technology cancontrol the generation of HIC to the low level, HIC still happened togenerate in some cases. In addition, a second problem is to cope withthe difficulty of completely inhibiting the generation of HIC even byapplying rigorous conditions in Ca treatment. The prior art has beenprimarily directed to optimization of Ca treatment conditions. However,though the Ca treatment conditions are rigorously managed in highstrength steel, there is still a problem in that the complete inhibitionof HIC generation is difficult.

Although the above-mentioned problems imply the possibility of thepresence of proper production conditions to be controlled other thanproper conditions for Ca treatment, their detailed contents andapproaches have been quite uncertain and solutions of these problems hasbeen difficult.

DISCLOSURE OF THE INVENTION

As described above, in conventional sour-resistant steel and theproduction method thereof, it is difficult to obtain stablesour-resistant steel, so that the establishment of stabilizationtechnique for sour-resistant steel has been a problem to be solved.Although the prior art has been mainly directed to the control ofCa-type inclusions and carbonitride-type inclusions, the control thereofis insufficient to obtain stable sour-resistant steel.

The present invention has been made in consideration of theabove-described problems, and a subject thereof is to provide a methodof producing steel for a steel pipe improved and stabilized insour-resistance performance by identifying the cause of generation ofHIC in terms of phenomena.

The present invention has been made to complete the above-describedsubject. The gist of the invention includes a method of producing steelfor a steel pipe excellent in sour-resistance performance shown in (1)to (4) below.

(1) A method of producing steel for a steel pipe excellent insour-resistance performance, the steel comprising, in % by mass, C, 0.03to 0.4%, Mn: 0.1 to 2%, Si: 0.01 to 1%, P: 0.015% or less, S: 0.002% orless, Ti: 0.2% or less, Al: 0.005 to 0.1%, Ca: 0.0005 to 0.0035%, N,0.01% or less, and 0 (oxygen): 0.002% or less, the balance being Fe andimpurities, in which the amount of Ca addition of Ca into molten steelin a ladle, where the non-metallic inclusions in the steel includes Ca,Al, O and S as main components, is controlled according to the N contentin the molten steel prior to addition of Ca such that the CaO content inthe inclusions is in the range of 30 to 80%, the ratio of the N contentin the steel to the CaO content in the inclusions satisfies the relationexpressed by equation (1) below, and the CaS content in the inclusionssatisfies the relation expressed by equation (2) below.

0.28≦[N]/(% CaO)≦2.0  (1)

(% CaS)≦25%  (2)

where [N] represents the mass content (ppm) of N in the steel, (% CaO)represents the mass content (%) of CaO in the inclusions, and (% CaS)represents the mass content (%) of CaS in the inclusions.

(2) The method of producing steel for a steel pipe excellent insour-resistance performance described in (1) above, the steel comprisingone or more elements selected from one or more of groups (a) to (c)below, in place of a part of Fe:

(a) in % by mass, Cr: 1% or less, Mo: 1% or less, Nb: 0.1% or less, andV: 0.3% or less;

(b) in % by mass, Ni: 0.3% or less, and Cu: 0.4% or less; and

(c) in % by mass, B: 0.002% or less.

(3) The method of producing steel for a steel pipe excellent insour-resistance performance described in (1) or (2) above, in which Cais added such that in controlling the amount of Ca addition into themolten steel in the ladle, the ratio of the N content in molten steel tothe amount of Ca addition to the molten steel satisfies the relationexpressed by equation (3) below according to the N content in the moltensteel prior to the Ca addition:

200≦[N]/WCA≦857  (3)

where [N] represents the mass content (ppm) of N in the molten steelprior to the Ca addition and WCA represents the amount of Ca addition(kg/t-molten steel) to the molten steel.

(4) The method of producing steel for a steel pipe excellent insour-resistance performance described in any one of (1) to (3) above, inwhich the molten steel is treated by the steps indicated by Steps 1 to 4below and then the above Ca is added in Step 5 below:

Step 1: CaO-type flux is added to molten steel in a ladle at atmosphericpressure;Step 2: after Step 1 above, the molten steel and the above CaO flux arestirred by injecting a stirring gas into the molten steel in the ladleat atmospheric pressure, and also an oxidizing gas is supplied to themolten steel to thereby mix the CaO-type flux with an oxide generated byreaction of the oxidizing gas with the molten steel;Step 3: the supply of the above oxidizing gas is halted anddesulfurization and the removal of inclusions are carried out byinjecting a stirring gas into the above molten steel in the ladle atatmospheric pressure;Step 4: an oxidizing gas is supplied into an RH vacuum chamber toincrease the molten steel temperature when the above molten steel in theladle is treated using an RH degasser after Step 3 above, andsubsequently the supply of the oxidizing gas is halted, and then thecirculation of the molten steel within the RH degasser is continued toremove inclusions in the molten steel; andStep 5: metallic Ca or a Ca alloy is added to the above molten steel inthe ladle after Step 4 above.

In the present invention, the term “non-metallic inclusions in the steelinclude Ca, Al, O, and S as main components” means that the total amountof these contents is 85% by mass or more. Small amounts of Mg, Ti, andSi may be included as other components.

In addition, “CaO-type flux” means the flux in which the CaO content is45% by mass or more and, for example, the flux mainly containing singlequicklime and quicklime-based flux containing components such as Al₂O₃and MgO are pertinent.

An “oxidizing gas” means a gas having the ability of oxidizing alloyingelements such as Al, Si, Mn and Fe in the melting temperature range ofsteel, whereas a single gas such as oxygen gas or carbon dioxide gas, amixed gas of these single gases and a blended gas of the above gaseswith inert gas or nitrogen are pertinent.

Additionally, in the descriptions below, the “in % by mass” representingthe constituent content is also simply expressed by “%”. Moreover, the“t-molten steel” representing one ton of molten steel is also simplyexpressed by “t”.

The present inventors have discussed a method of producing steel for asteel pipe improved and stabilized in sour-resistance performance tocomplete the foregoing subject, obtained findings described below, andcompleted the above-described present invention.

1. Chemical Composition of Steel for a Steel Pipe and Inclusions inSteel

1-1. Chemical Composition of Steel for Steel Pipe

As described above, conventionally, even if the improvement ofcleanliness of steel and the morphology control of Ca-type inclusionsor, in addition thereto, the increase of strength bydispersion/deposition of carbonitrides was attempted, there still existsmany unidentified causes of rendering sour-resistance performanceunstable. This fact suggests that sour-resistance performance maydeteriorate due to causative factors other than oxysulfides or sulfidesincluding Ca-type inclusions, MnS and CaS, or bulky TiN.

Thus, the present inventors have fully investigated the initiation pointof HIC. First described is the reason why the present invention islimited to such a steel composition that comprises C, 0.03 to 0.4%, Mn:0.1 to 2%, Si: 0.01 to 1%, P: 0.015% or less, S: 0.002% or less, Ti:0.2% or less, Al: 0.005 to 0.1%, Ca: 0.0005 to 0.0035%, N, 0.01% orless, and (oxygen): 0.002% or less, and further, where needed, comprisesone or more of elements selected from a group consisting of Cr: 1% orless, Mo: 1% or less, Nb: 0.1% or less, V: 0.3% or less, Ni: 0.3% orless, Cu: 0.4% or less, and B: 0.002% or less, the balance being Fe andimpurities.

C, 0.03 to 0.4%

C has a function that improves the strength of steel, and is anindispensable constituent element. If the C content is less than 0.03%,a sufficient strength for the steel is not obtained. On the other hand,if the content exceeds 0.4% and becomes high, hardness becomes too highand thus the cracking susceptibility is increased, so that thegeneration of HIC cannot be sufficiently suppressed. Hence, the properrange of the C content was set to be from 0.03 to 0.4%. The C contentpreferably ranges from 0.05 to 0.25%.

Mn: 0.1 to 2%

Mn is also an indispensable element to improve the strength of steel. Ifthe Mn content is less than 0.1%, a sufficient strength for the steel isnot obtained. On the other hand, if its content exceeds 2% and becomeshigh, inhibiting the generation of MnS becomes difficult and, at thesame time, the compositional segregation becomes notable. Hence, theproper range of the Mn content was set to be from 0.1 to 2%. Thepreferred range of the content is from 1.2 to 1.8%.

Si: 0.01 to 1%

Si not only functions as a deoxidizing element, but affects activitiesof Ti and Ca in steel. Therefore, if Si content is less than 0.010, theCa activity cannot be increased, while if its content exceeds 1% andbecomes high, the Ti activity is increased too much, whereby thegeneration of TiN cannot be suppressed. Accordingly, the proper contentrange of Si is from 0.01 to 1%. The preferred range of the content isfrom 0.1 to 0.5%.

P: 0.015% or Less

P is an element that heightens cracking susceptibility since itsegregates in steel and increases hardness of steel in a segregationportion. Therefore, the content needs to be set to 0.015% or less. Onthe other hand, reducing the P content to less than 0.005% leads to anincrease in refining costs, so that its content is preferably 0.005% ormore from economical aspect.

S: 0.002% or Less

Since S is a constituent element of sulfide-type inclusions that pose aproblem in HIC resistant steel, its content is preferably low. If the Scontent exceeds 0.002% and becomes high, the CaS content in theinclusions becomes high when Ca is added, whereby the relationshipbetween the CaO content and the N content in the inclusions as describedbelow is difficult to be satisfied. Thus, the S content needs to be0.002% or less. The preferred range of the content is 0.001% or less.

Ti: 0.2% or Less

Ti is an element that precipitates in steel as TiN and has the functionof improving toughness of steel. However, excessive addition of Ti leadsto the coarsening of TiN to be precipitated. Thus, the Ti content needsto be 0.2% or less. Its content is preferably set to be 0.005% or morefrom the viewpoint of securing toughness. From the above reasons, the Ticontent is preferably 0.005% or more and needs to be 0.2% or less.

Al: 0.005 to 0.1%

Al is an element that has strong deoxidization effect and an importantelement for lowering an oxygen content in steel. Its content of lessthan 0.005% is insufficient for deoxidization effect and cannotsufficiently decrease the amount of inclusions. On the other hand, whenthe Al content exceeds 0.1% and becomes high, the generation of sulfidesis aggravated in addition to the saturation of the deoxidization effect.Hence, the proper range of the Al content was set to be from 0.005 to0.1%. The preferred range of the content is from 0.008 to 0.04%.

Ca: 0.0005 to 0.0035%

Ca is an element that exerts effective action for reforming sulfideinclusions and spheroidizing alumina inclusions. When the Ca content isless than 0.0005%, these effects cannot be obtained and thus thegeneration of HIC attributable to MnS or alumina clusters cannot besuppressed. On the other hand, when the content exceeds 0.0035% andbecomes high, a CaS cluster may be generated. Hence, the proper range ofthe Ca content was set to be from 0.0005 to 0.0035%. The contentpreferably ranges from 0.0008 to 0.002%.

N, 0.01% or Less

N is an element that constitutes bulky TiN, so that its content ispreferably low. When the N content exceeds 0.01% and becomes high, thegeneration temperature of TiN rises and becomes near a steel refiningtemperature or a casting temperature, so that the coarsening of TiNcannot be restrained. Hence, the proper range of the N content was setto be 0.01% or less. On the other hand, its content is preferably0.0015% or more from an economical viewpoint. Moreover, its content ispreferably 0.005% or less to particularly improve toughness.

O (Oxygen): 0.002% or Less

The O content means the total oxygen content (T. [O]) that includes theoxygen contained in oxide-type inclusions and serves as a measure of theamount of inclusions. When this content exceeds 0.002% and becomes high,the amount of inclusions becomes too big and the suppression ofgeneration of HIC in high-strength steel becomes difficult. The lowerthe O content, the smaller the amount of oxide-type inclusions. However,its content is preferably set in the range of 0.0003 to 0.0015% in orderto readily satisfy the relationship between the CaO content ininclusions described below and the N content in steel.

The above covers essential compositional elements in steel for a steelpipe and their composition ranges in the present invention, and one ormore of elements selected from one or more of groups out of (a) to (c)listed below can be contained according to applications and useenvironments of steel. In other words, Group (a) includes Cr, Mo, Nb andV; Group (b) includes Ni and Cu; and Group (c) includes B. Elements ofeach of the above groups may or may not be contained. However, ifcontained, they can be each contained in the content ranges as below toexhibit their effects.

The elements of Group (a) are Cr, Mo, Nb and V, and have the function ofimproving strength or toughness of steel.

Cr: 1% or Less

Cr is an element having a function that improves strength of steel. Whenits effect is pursued by containing Cr, including 0.005% or more enablesthe above effect to be exhibited. However, if its content exceeds 1% andbecomes high, the toughness of the welded portion is decreased.Accordingly, when Cr is to be contained, its content may be in the rangeof 1% or less. In addition, the Cr content is preferably 0.005% or more.

Mo: 1% or Less

Mo is also an element having a function that improves strength of steel.When its effect needs to be pursued, including 0.01% or more thereofmakes it possible to exhibit the above effect. However, if its contentexceeds 1% and becomes high, weldability is worsened. Thus, if needed,Mo may be included in the range of 1% or less. Moreover, its content ispreferably set in the range of 0.01% or more.

Nb: 0.1% or Less

Nb is an element that has the effect of improving toughness bygrain-refining of a steel structure. Including 0.003% or more thereofcan exhibit its effect. However, if its content exceeds 0.1% and becomeshigh, the toughness of a welded portion is decreased. Thus, if needed,Nb may be included in the range of 0.1% or less. In addition, itscontent is preferably made 0.003% or more.

V: 0.3% or Less

V is also an element that has the effect of improving toughness bygrain-refining of a steel structure. Containing V of 0.01% or moreenables its effect to be exhibited. However, if its content exceeds 0.3%and becomes high, the toughness of a welded portion is decreased. Thus,if needed, V may be included in the range of 0.3% or less. Moreover, itscontent is preferably 0.01% or more.

The elements of Group (b) are Ni and Cu, and have the function ofsuppressing the intrusion of hydrogen in a hydrogen sulfide environment.

Ni: 0.3% or Less

Ni has the function of suppressing the ingress of hydrogen into steel ina hydrogen sulfide environment. When its effect needs to be pursued,containing 0.1% or more of Ni makes it possible to exhibit the aboveeffect. However, since, when its content exceeds 0.3% and becomes high,the effect of suppressing the hydrogen ingress is saturated, the Nicontent may be set 0.3% or less. In addition, its content is preferablyset in the range of 0.1% or more.

Cu: 0.4% or Less

Cu also has the function of suppressing the ingress of hydrogen intosteel in a hydrogen sulfide environment similarly to Ni. When its effectneeds to be pursued, containing 0.1% or more of Cu makes it possible toexhibit the above effect. However, since, when its content exceeds 0.4%and becomes high, the steel melts at high temperature, which decreasesthe strength of grain boundary, if Cu is needed, its content may be setto 0.4% or less. In addition, its content is preferably set in the rangeof 0.1% or more.

The element of Group (c) is B and has the function of improvinghardenability of steel.

B: 0.002% or Less

B is an element that has the effect of improving hardenability of steel.When its effect needs to be pursued, containing 0.0001% or more of Bmakes it possible to exhibit the above effect. However, since, when itscontent exceeds 0.002% and becomes high, the hot workability of steel islowered, if B is needed, its content is set to 0.002% or less. Moreover,its content is preferably made in the range of 0.0001% or more.

1-2. Chemical Composition of Inclusions in Steel

The reasons why the composition of inclusions mainly comprises aCa—Al—O—S system and a CaO content in inclusions is limited to 30 to 80%will be described.

The presence of Ca—Al—O-type inclusions is indispensable to restrain thegeneration of MnS, despite that Ca is added to restrain the generationof MnS. In addition, if Ca is not contained, alumina cluster inclusionsare formed and become an initiation to generate HIC in some cases.Hence, in the present invention, inclusions were configured to mainlycomprise a Ca—Al—O—S system. However, a small amount of MnS, SiO₂ andcarbonitrides might be generated on surfaces of Ca—Al—O-type inclusionsdue to composition segregation and temperature decrease duringsolidification. This does not affect the generation of HIC and thus doesnot particularly need to be limited.

Next, the range of the CaO content in inclusions will be described. Whenthe CaO content becomes less than 30%, the effect of suppressing thegeneration of MnS is lowered, and in addition, the melting point ofinclusions is increased, thereby likely inducing the clogging of castingnozzles, whereby it becomes difficult to secure stable productivity.

On the other hand, if the CaO content in inclusions exceeds 80% andbecomes high, the solid phase ratio in inclusions at a molten steeltemperature is risen to thereby make it impossible to maintain aspherical shape in inclusions. On account of this, the Ca—Al—O-typeinclusions result in a massive or angular shape, which may become aninitiation of the generation of HIC.

From the above reasons, the proper range of the CaO content ininclusions was specified in the range of 30 to 80%.

In the present invention, steel compositions were limited as describedabove, and the relationship between inclusions and the generation of HICwas investigated within the respective content ranges.

1-3. Investigation of Relationship Between Inclusions in Steel andGeneration of RIC

200 kg of molten steel was made and adjusted it within the range of theabove composition and then tapped into a mold to yield a steel ingot. Atest piece was cut out of the resulting steel ingot, and inclusions inthe steel was closely observed. Asa result, as described in the aboveWO2005/075694, bulky TiN was decreased by addition of Ca and thegeneration of TiN around the Ca—Al—O-type inclusions was observed.Additionally, when no addition of Ca, it was ascertained that many bulkyTiN inclusions were generated and at the same time MnS was generated aswell.

Moreover, Ca—Al inclusions appear in a spherical shape and neitheroxide-type clusters nor CaS clusters were generated. When minusculeinclusions were observed, as described in Japanese Patent ApplicationPublication No. 2003-313638, extremely tiny carbonitrides that areconsidered not to be pertinent to the generation of HIC were alsoobserved. These results well agree with the results disclosed in theprior art and indicate the validity of the present investigation. Asstated above, a variety of inclusions are generated in thesour-resistant steel, the prior art has been directed to mainlycontrolling these inclusions.

Next, the dispersion states of various inclusions were investigated. Asa result, it has been shown that, when Ca is added, Ca-containingoxysulfide-type inclusions are uniformly dispersed, while fortitanium-type carbonitrides with a relatively small size of 1 to 10 μm,there exist two patterns, one is that they are uniformly dispersed, theother is that several to tens of them are aggregated/overcrowded withina square area of about 30 to 70 μm in side length. The present inventorshave paid attention to titanium-type carbonitrides present in theaggravated state (hereinafter, also noted as a “collectivecarbonitrides”).

The above collective carbonitrides are comprised of tiny carbonitridesof 30 μm or less in size and it is presumed that such a single tinycarbonitride would not lead up to the generation of HIC by virtue ofthis size. However, it is considered that, when these inclusions areaggregated and appear in a narrow region, the collective carbonitridesbehave like a single inclusion, thereby possibly affecting thegeneration of HIC.

Fundamentally, where this collective carbonitrides cause the generationof HIC, it is important to quantify and evaluate this size. However,small carbonitrides are considered to gather three-dimensionally to formthis collective carbonitrides, so that there is a problem in that thesize flatly observed does not necessarily correspond to the size of thecollective carbonitrides.

Hence, the present inventors discussed a measure that can specify thestate of collective carbonitrides with further higher precision. When asingle carbonitride of 1 to 10 μm is present in the range of tens of μmwithout dependency on the size, one collective carbonitrides were judgedto be present and the number of collective carbonitrides present on thesurface of a test piece of 30 mm×30 mm was measured. As a result, whenthe number of collective-carbonitride-type inclusions is represented bythe N content in steel and the CaO content in the Ca—Al-type oxysulfideinclusions, a correlation was found between HIC resistance performanceand the contents.

As described above, though the size or the number of sets ofcarbonitrides lacks precision, the N content in steel and the CaOconcentration in Ca—Al-type oxysulfide inclusions can be determined withhigh precision. In addition, it is considered that when the N content insteel is high, the generation of the carbonitride is promoted, so thatthe number of sets of carbonitrides increases and the size also becomeslarge. Additionally, it is speculated that a proper range in the CaOcontent in the inclusions is present to generate carbonitrides onsurfaces of Ca—Al-type inclusions. Then, the present inventors haveconsidered that the behavior of collective carbonitrides can be analyzedfrom the ratio of the N content in the steel to the CaO content in theinclusions, or the value of [N]/(% CaO), on the basis of the aboveresults.

Accordingly, 180 kg of molten steel was adjusted to the above steelcomposition, the strength of the resulting steel ingot is adjusted tothe X80 grade of API Standards, and then the HIC resistance performancewas evaluated according to the method stipulated in NACE (NationalAssociation of Corrosion Engineers) TM0284-2003. Specifically, 10 testpieces each being 10 mm thick×20 mm wide×100 mm long were sampled fromeach steel ingot thus made, and these were immersed in an aqueoussolution (0.5% acetic acid+5% salt) at 25° C. saturated with hydrogensulfide at 1.013×10⁵ Pa (1 atm). The area of HIC generated in each testpiece after testing was measured by ultrasonic flaw detection, and thenthe crack area ratio (CAR) was obtained by equation (4) below. Here, thearea of the test piece in equation (4) was set to be 20 mm×100 mm.

Crack area ratio (CAR)=(total value of area of HIC generated in testpiece/tested area of test piece)×100(%)  (4)

In this regard, it was judged that the case where the crack area ratio(CAR) was less than 1% was taken as no generation of HIC and that thecase where CAR was 1% or more was taken as generation of HIC.

FIG. 1 shows the relationship between [N]/(% CaO) that is the ratio ofthe N content in steel to the CaO content in inclusions and the numberof collective carbonitrides. In addition, FIG. 2 shows the relationshipbetween [N]/(% CaO) that is the ratio of the N content in steel to theCaO content in inclusions and the generation rate of HIC. The results inthese FIGS. 1 and 2 are ones that are obtained by examination of steeltypes of X70 grade in API Standards. Additionally, the generation rateof HIC in FIG. 2 was indicated by the ratio of the number of test piecesthat generated HIC out of 30 test pieces sampled from the same steelcomposition. For example, when HIC is generated in one test piece out of30 test pieces, the generation rate of HIC is 3.33%.

FIG. 1 shows that, when the CaS content in inclusions is 25% or less,collective carbonitrides are not generated if [N]/(% CaO) as being theratio of the N content in steel to the CaO content in inclusions iswithin the range of 0.28 to 2.0 (ppm/% by mass). As a result, as shownin FIG. 2, HIC is completely suppressed when the ratio of the N contentin steel to the CaO content in inclusions is within the range of 0.28 to2.0 (ppm/% by mass). However, when the CaS content in inclusions exceeds25% and becomes high, the generation of the collective carbonitrides isnot suppressed, as shown in FIG. 1, even if the value of [N]/(% CaO) iswithin the range of 0.28 to 2.0 (ppm/% by mass). As a result, as shownin FIG. 2, HIC is apparently generated.

In other words, it has become apparent that the relations represented byequations (1) and (2) below need to be satisfied at the same time tosecure HIC resistance performance in high strength steel.

0.28≦[N]/(% CaO)≦2.0  (1)

(% CaS)≦25%  (2)

The above results are indicative that when the N content in steel is toohigh or when the CaO content in inclusions is not present within aproper range and the two are not properly balanced, the generation ofcollective carbonitrides cannot be suppressed to thereby cause HIC to begenerated. Moreover, it is speculated that CaS tends to be generated onthe surface of any of Ca—Al-type oxysulfide inclusions when the CaScontent in inclusions exceed 25% and becomes high, thereby inhibitingthe generation of carbonitrides onto the surface of any of Ca—Al-typeoxysulfide inclusions, resulting in promoting the generation ofcollective carbonitrides.

The inventions according to claims 1 and 2 to secure HIC resistanceperformance in high strength steel have been completed on the basis ofthe findings described in 1-1. to 1-3. above.

2. Balance Between N Content in Molten Steel and Amount of Ca Addition

As described above, properly adjusting the balance between a chemicalcomposition in inclusions and the N content in steel enables to suppressthe generation of RIC better than the case in the prior art by. Now,further, a method of more simply and easily obtaining the above type ofinclusions will be described. In the present invention, the CaO contentin inclusions is controlled by the amount of Ca addition. Besides, thereis a need to balance the amount of Ca addition with the N content inmolten steel since it is necessary to adjust the balance between the Ncontent in steel and the CaO content in inclusions.

Then, the N content in molten steel prior to Ca addition and the amountof Ca addition were varied using 10 kg of molten steel to therebyinvestigate the relationship between [N]/WCA as being the ratio of thetwo and [N]/(% CaO) as being the ratio of the N content in steel to theCaO content in inclusions. The testing was repeated 4 times and itsresults were evaluated.

FIG. 3 is a diagram indicating the relationship between [N]/WCA and N/(%CaO). In the diagram, [N] in relation to [N]/WCA represents the Ncontent in molten steel (ppm) prior to Ca addition and WCA representsthe amount of Ca addition per production unit (kg/t-molten steel) intomolten steel.

As indicated in the results of FIG. 3, all four tests satisfied therange of [N]/(% CaO) specified in claim 1 in the range in which thevalue of [N]/WCA is from 200 to 857 (ppm %/kg). On the other hand, inthe range in which the value of [N]/WCA is outside the above, there werecases where some satisfy and the others cannot satisfy the range of[N]/(% CaO) specified in claim 1. From the above results, if the valueof [N]/WCA satisfies the conditions expressed by equation (3) below, thevalue of [N]/(% CaO) satisfies the relation of equation (1) abovespecified in claim 1, and therefore, steel for a steel pipe can bestably produced by the production method according to claim 1.

200≦[N]/WCA≦857  (3)

3. Step of Producing Steel for Steel Pipes

The invention according to claim 4 is an invention that specifies a stepof producing steel for a steel pipe. The reason of the limitation foreach step will be described in the following. In the present invention,the lower and more stable the N content in molten steel, the more thecontrollability of inclusions is improved to make it easy to producesteel for a steel pipe by a production method according to claim 1. Inaddition, the lower and more stable the N content in molten steel, themore the amount of Ca addition can be decreased and the less theproduction cost can be and at the same time the less the variation ofthe amount of Ca addition in each treatment can be. Furthermore, as theamount of inclusions in molten steel is lowly stable, the above effectsincrease all the better. Additionally, the lower the S content in moltensteel, the easier the relation of equation (2) specified in claim 1 issatisfied.

Therefore, it is important to optimize melting and refining process ofsteel and to stabilize cleanliness and the N content in steel in orderto further stably produce steel for a steel pipe of the presentinvention.

In other words, the invention according to claim 4 is a method ofrefining steel for a steel pipe that promotes desulfurization andpurification as well as lowering the N content at the same time tothereby allow the invention according to any of claims 1 to 3 to becarried out efficiently and stably by controlling thetemperature-raising process of molten steel as well as by optimizing thestirring treatment of molten steel and slag.

An optimal process in the present invention comprises following Steps 1to 5:

Step 1: CaO-type flux is added to molten steel in a ladle at atmosphericpressure;Step 2: after Step 1 above, the molten steel and the above CaO flux arestirred by injecting a stirring gas into the molten steel in the ladleat atmospheric pressure, and also an oxidizing gas is supplied to themolten steel to thereby mix the CaO-type flux with an oxide generated bythe reaction of the oxidizing gas with the molten steel;Step 3: the supply of the above oxidizing gas is halted anddesulfurization and the removal of inclusions are carried out byinjecting a stirring gas into the above molten steel in the ladle atatmospheric pressure;Step 4: an oxidizing gas is supplied into an RH vacuum chamber toincrease the molten steel temperature when the above molten steel in theladle is processed using an RH degasser after Step 3 above, andsubsequently the supply of the oxidizing gas is halted, and then thecirculation of the molten steel within the RH degasser is continued toremove inclusions in the molten steel; andStep 5: metallic Ca or a Ca alloy is added to the above molten steel inthe ladle after Step 4 above.

In order to melt and refine an extra-low-sulfur high-cleanliness steelthat simultaneously achieves extra-low-sulfur and high purification asdescribed above, treatments and processing in Steps 1-5 are effective asdescribed in 3-1. to 3-5 below.

When Al and oxygen are supplied to molten steel, the molten steeltemperature is raised and also Al₂O₃ is generated. This Al₂O₃ floats tothe surface of molten steel with increasing molten steel temperature andis absorbed into slag after floating. At this time, the Al₂O₃ and slagintegrate with each other at high temperature and the absorption of theAl₂O₃ into this slag changes the chemical composition of the slag.Further, Al₂O₃ is gradually generated with supply of oxygen andsequentially gets surfaced, and thus a change in the chemicalcomposition of the slag is gradual; a rapid composition change of theslag, which takes place in the case where Al₂O₃ or synthetic flux isadded, does not occur. Furthermore, since Al₂O₃ uniformly floats to theentire molten steel surface, it disperses in the entire slag. And thiscase is different from a local addition as in a batch addition, wherebythe slag can be sufficiently stirred and mixed even if the stirring isweak and also the mixing time can be shortened.

Therefore, the slag chemical composition can be controlled by utilizingthe Al₂O₃ component generated by supply of Al and oxygen to molten steelfor the control of a slag chemical composition to attempt to mix theAl₂O₃ component at high temperature, to gradually change the compositionand to uniformly disperse the Al₂O₃ component. The control of thechemical composition of the slag described above makes it possible toavoid strong stirring and also shorten the treatment time, so that otherthan desulfurization achievement, an increase in the N content in moltensteel by nitrogen absorption from air can be suppressed.

3-1. Step 1

In Step 1, the CaO-type flux is added to molten steel at atmosphericpressure to undergo desulfurization. Here, the reason of CaO addition atatmospheric pressure is that since CaO addition under reduced pressureincreases refining costs in Step 1 and oxidation refining is carried outin the subsequent step, it is unnecessary to do it under reducedpressure. Though Al is basically supplied to molten steel prior toaddition of the CaO-type flux, it may be added at the same time with theaddition of the CaO-type flux. Nitrogen absorption from air can besuppressed by slag by addition of Al in the earliest stage of CaOtreatment, in addition to the improvement of desulphurizationefficiency.

3-2. Step 2

Next, in Step 2, the molten steel and the added flux are stirred byinjecting an inert gas into the molten steel in the ladle at atmosphericpressure and also an oxidizing gas is supplied to the molten steel tothereby mix the CaO-type flux with an oxide generated by the reaction ofthe oxidizing gas with the molten steel. This treatment is to react theAl in the molten steel with oxygen and utilize the generated Al₂O₃component to thereby control the chemical composition of the slag andpromote melting of the slag. Here, the reason why an inert gas isinjected thereinto is that the absorption of an oxidizing gas intomolten steel smoothly proceeds by virtue of the inert gas injection.This is because, when an oxidizing gas only is supplied withoutinjecting an inert gas thereinto, oxidation reaction progresses only inthe limited region where the oxidizing gas collides with the moltensteel surface, and the homogeneous distribution of Al₂O₃ is retarded.

In Step 2, as the control of a slag chemical composition and its meltingprogress, the effect of inhibiting nitrogen absorption from air isincreased by this melting, and the desulfurization reaction proceeds atthe same time. However, the desulfurization reaction does not reach thesaturated state within the time period for supplying the oxidizing gasmentioned above and a desulfurizing capability surplus remains in theslag. Here, “desulfurizing capability surplus” means desulfurizingability governed by the chemical composition of slag as described below.In addition, Al₂O₃ remains in the molten steel by an amount of tens ofppm as inclusions though it is not large enough to change the chemicalcomposition of the slag.

3-3. Step 3

Thus, after Step 2 above, the supply of an oxidizing gas is halted inStep 3, and desulfurization and removal of inclusions are performed byinjecting a stirring gas into the molten steel at atmospheric pressure.By this treatment, further desulfurization with slag havingdesulfurizing capability surplus and removal of unwanted residualinclusions are attempted. “Desulfurizing capability surplus” here meansthe sulfide capacity governed by the chemical composition of slag, thatis, the “desulfurizing capability”. This sulfide capacity lowers iflower grade oxides such as FeO and MnO are present in slag. Therefore, aslag chemical composition should be controlled to decrease theconcentration of lower grade oxides to exhibit desulfurizing power toits maximum.

In Step 2 as above, the supply of an oxidizing gas inevitably generateslower grade oxides. On account of this, an inert gas is injected in Step3 after Step 2 to reduce the concentration of these lower grade oxides,thereby further enabling desulfurization to be promoted. Additionally,slag can be sufficiently melted in Steps 1 and 2, whereby nitrogenabsorption from air can be suppressed even if the inert gas is injectedand stirring is carried out.

3-4. Step 4

Next, Step 4 is conducted. In Steps 1 to 3 above, molten steel in theladle is treated at atmospheric pressure. After these treatments, theladle is transferred to RH vacuum degassing equipment (hereinafter, alsonoted as “RH equipment” and treatment by RH equipment is also noted as“RH treatment”), and an oxidizing gas is supplied to the molten steel inRH treatment to increase the molten steel temperature. In addition, themolten steel is then circulated in the RH equipment. Treatments in thisstep can further improve the desulfurization efficiency and cleanliness.

The reason is as follows. That is to say, the temperature can be raisedalso in Step 2 as above, and its main object is to promotedesulfurization by controlling the chemical composition of slag. Becauseof this, even when the molten steel temperature is too low, the amountof temperature increase of the molten steel by oxygen supply may belimited. For example, when the molten steel temperature before treatmentis lower than a specific planned value, the amount of supply of anoxidizing gas needs to be increased to raise the molten steeltemperature. However, since the amount of formation of Al₂O₃ increaseswhen the oxidizing gas supply amount is increased, the amount ofintroduction of CaO cannot help being increased. This results in anincrease in the amount of slag.

Thus, the following method was adopted in the present invention. Inother words, the amount of supply of an oxidizing gas in Step 2 is takenas the amount of supply of oxygen suitable for the control of thechemical composition of slag that is primarily directed todesulfurization. In this case, the molten steel temperature may becomeslightly low. This temperature shortage should be compensated in any ofthe stages. As described above, when the temperature is increased usingan oxidizing gas, the concentrations of FeO and MnO in the slag areincreased, resulfurization from the slag to the molten steel couldpossibly happen. Accordingly, we paid attention to the fact that almostno reaction between the slag and the molten steel proceeds in the RHtreatment.

The reaction between the slag and the molten steel in RH treatment isslow, so that the resulfurization is not easily caused even if the FeOand the MnO contents or the Al₂O₃ content is increased in the slagduring RH treatment. Therefore, when the molten steel temperature isinsufficient in Step 2, the molten steel temperature may be increased bysupplying an oxidizing gas in Step 4, RH treatment. This method canimprove desulfurization effects in Steps 1 to 3 and further compensatethe molten steel temperature without spoiling the desulfurizationeffects.

In addition, the implementation of RH treatment after each treatment atatmospheric pressure makes it possible to carry out denitrificationtreatment in the end and further obtain nitrogen-decreasing effect.

Additionally, though the purification effect of molten steel is obtainedby treatment of Step 3 above, when cleanliness higher than that obtainedby Step 3 is demanded, cleanliness can be improved by further continuingto circulate molten steel in RH equipment after the supply interruptionof an oxidizing gas. Besides inclusions partly remaining even aftertreatment of Step 3, when the molten steel temperature is adjusted bycarrying out temperature-raising heating while the desulphurizationefficiency is kept high-level in. Step 4, Al₂O₃ inclusions may begenerated by temperature-raising heating to remain in the molten steel.In such case, to remove these inclusions, the cleanliness of moltensteel can be still further improved by performing circulation treatmentfor a fixed time after supply of an oxidizing gas.

3-5. Step 5

Finally, Ca is added to the molten steel in Step 5. The S and N contentsin the molten steel are stable at a low level and the cleanliness isalso high by treatments of Steps 1 to 4, whereby the method of producingsteel for a steel pipe described in claim 1 or 2 can be stably carriedout by addition of Ca in step 5. In this case, the amount of Ca additionis more preferably set in the range that satisfies the relation ofequation (3) specified in claim 3.

A rise in temperature of molten steel and control of the chemicalcomposition of slag can be performed simultaneously to increase thecleanliness of the steel as well as to reduce sulfur and nitrogen bycarrying out the treatment by Steps 1 to 5 described as above in theorder numbered.

3-6. Confirmation of Effectiveness of Invention

The present inventors conducted the following tests and confirmed theeffectiveness of the invention according to claim 4. Using 250 tons (t)of molten steel having chemical compositions indicated in Table 1, TestsE1 to E6 are carried out, the outlines of which were shown below.

TABLE 1 Chemical composition (% by mass) C Si Mn P S Al N T. [O]0.04~0.06 0.1~0.3 0.5~1.2 0.007~0.010 0.0028~0.0035 0.01~0.030.0030~0.0045 0.0035~0.0055

Test E1: Steps 1, 2, 3 and 5 only were carried out.

Test E2: Steps 1, 2, 4 and 5 only were carried out.

Test E3: Steps 2, 3, 4 and 5 were sequentially carried out after Step 2.

Test E4: Steps 1, 2, 3 and 5 were sequentially carried out after Step 4.

Test E5: Steps 4 and 5 only were carried out.

Test E6: It was carried out as in claim 4.

Detailed conditions in each step were set in the following. That is, theamount of CaO to be added in Step 1 was set at 8 kg/(t-molten steel) andadded to molten steel immediately after the start of treatment. In Step2, an Ar gas was injected into molten steel at a flow rate of 0.01 Nm³/tat atmospheric pressure and at the same time an oxygen gas was sprayedonto the molten steel surface at a feed speed of 0.16 Nm³/(min·t) for 10minutes. In Step 3, the flow rate of an Ar gas was set at 0.01 Nm³/t andstirring treatment was performed for 10 minutes.

In addition, in Step 4, an oxygen gas was sprayed onto the molten steelsurface within the RH vacuum chamber for 3 minutes at a feed rate of0.14 Nm³/(min·t), and then the molten steel was circulated for 10minutes. Then, in Step 5, a CaSi alloy was added according to therelation of equation (3) above depending on the N content in the moltensteel analyzed in Step 4. Additionally, the amount of Ca addition (WCA)in equation (3) indicates genuine metal Ca to be added (kg/t-moltensteel) in terms of the mass per production unit, and therefore theamount of addition of the CaSi alloy was controlled such that the massof genuine metal Ca in the CaSi alloy satisfied the relation of equation(3) .

The results of the S and N contents, cleanliness indexes, minima andmaxima [N]/(% CaO) obtained by above Tests were shown in Table 2.

TABLE 2 Test [S] [N] Cleanliness Minimum Maximum No. (ppm) (ppm) index[N]/(% CaO) [N]/(% CaO) E1 4 48 1.8 0.45 1.80 E2 3 39 1.7 1.10 1.70 E315 51 2.1 1.20 1.70 E4 13 62 1.7 0.70 1.80 E5 25 35 1.9 0.80 1.70 E6 338 1.0 1.30 1.50

In this Table, the cleanliness index was indicated by setting the numberof inclusions in Test E6 to 1.0 as norm. Moreover, the minimum [N]/(%CaO) and the maximum [N]/(% CaO) indicated respectively the minimumvalue and the maximum value of 25 inclusions for each Test that wereexamined.

Though, from the results of the Table, various processes are possibleaccording to steps to be adopted and their combinations, it has beenascertained that the variation of the values of N/(% CaO) is thesmallest for Test E6 according to the invention described in claim 4.The above results clearly indicated that the method of treating moltensteel by processes indicated in Steps 1 to 5 as described in claim 4 isa melting and refining method that can control the inclusions with thehighest precision that is intended by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram indicating the relationship between [N]/(% CaO) asbeing the ratio of the N content in steel to the CaO content ininclusions and the number of collective carbonitrides.

FIG. 2 is a diagram indicating the relationship between [N]/(% CaO) asbeing the ratio of the N content in steel to the CaO content ininclusions and the generation rate of HIC.

FIG. 3 is a diagram indicating the relationship between [N]/WCA as beingthe ratio of the N content in steel to the amount of Ca addition and[N]/(% CaO).

BEST MODE FOR CARRYING OUT THE INVENTION

The composition other than Ca in steel for a steel pipe of the presentinvention may be adjusted between before the addition of Ca and afterthe completion of converter blowing. In particular, they are preferablyadjusted before the processes of Steps 1 to 4 described in claim 4 arecompleted. The reason is that, when the composition is adjusted afterthe addition of Ca, the treatment time period of molten steel becomeslong, and during its time period, the Ca evaporates and thus the Cacontent in the steel is unpreferably significantly lowered.

1. Best Mode of Inclusions in Steel

In the present invention, nonmetallic inclusions in steel areCa—Al—O—S-type inclusions by addition of Ca to the steel compositiondescribed in claim 1. The inclusions primarily include CaO—CaS—Al₂O₃ andgenerate carbonitrides including Ti, Nb, etc. on their surfaces. Thiscarbonitrides may be generated either on the surfaces of Ca—Al—O-typeinclusions in film form or partially on their surfaces. In addition, thecontent of the carbonitrides generated on the surfaces is notparticularly specified. Moreover, MnS may be generated on the surfacesof the inclusions by composition segregation, and this does notparticularly affect HIC.

However, the CaO content in the inclusions needs to be from 30 to 80%.Preferably, the CaO content in the inclusions is from 45 to 60%. Thisreason is that CaO can be spheroidized more stably than inclusions,while allowing wettability with molten iron to be improved to therebypromote the generation of carbonitrides onto the surfaces of theinclusions.

The CaS content in the inclusions may be 25% or less, preferably 15% orless, more preferably 5% or less. This is because the lower the CaScontent, the more the generation of carbonitrides onto the surfaces ofthe Ca—Al—O—S-type inclusions is facilitated and at the same time theability of capturing S as segregation element during solidification ispromoted.

In addition, when the Al content in steel is 0.008% or less, oxides ofSi or Ti may be generated on the surfaces of Ca—Al—O—S-type inclusions;however, this does not particularly affects HIC. However, this leads tothe enlargement of inclusions, so that oxides of Si or Ti are preferablytotally 15% or less.

2. Best Mode of Ca Addition

In the present invention, the composition of inclusions during arefining step does not need to be identified, it is enough to performquick analysis prior to the Ca addition to measure the N content insteel and determine the amount of Ca addition based on the measurementresult and equation (3) above. Here, WCA in equation (3) is the genuineadded metallic Ca per production unit, i.e., the genuine mass of Ca in aCa-containing agent added to one (1) ton of molten steel (kg/t-moltensteel).

For instance, when a CaSi alloy having a Ca content of 35% and a Sicontent of 65% is added in a proportion of 1 kg/(t-molten steel), WCA is0.35 kg/(t-molten steel). Incidentally, the addition of metallic Ca isconcerned, so that, for example, when a mixture having 50% of Ca and 50%of CaO is added in an amount of 1 kg/(t-molten steel), WCA is 0.5kg/(t-molten steel).

Here, Ca agents to be added that can be used include, in addition tometallic Ca, alloys such as CaSi and CaAl or mixtures of the abovealloys and compounds like CaO, Al₂O₃, and the like.

A method of adding can be any one such as an injection method thatinjects Ca additives into molten steel together with carrier gas, amethod of making Ca additives in the form of wire or feeding wireshavingCa additives embedded inside into molten steel, or the like. However,the addition rate is preferably in the range of 0.01 to 0.1kg/(min·t-molten steel) in terms of genuine metallic Ca. The reason isthat, when the addition rate is less than 0.01 kg/(min·molten steel),the treatment time gets too long, while when the addition rate exceeds0.1 kg/(min·t-molten steel) and becomes high, splashing and the likebecomes violent.

Moreover, the value of WCA as Ca addition amount is preferably made tobe in the range of 0.05 to 0.25 kg/t-molten steel). If the value of WCAis less than 0.05 kg/(t-molten steel), the distribution of CaOconcentrations in inclusions could be very likely to be in a lowerlevel, while if the value of WCA exceeds 0.25 kg/(t-molten steel) andbecomes high, the oxygen activity becomes too low to thereby getnitrogen absorbed to increase the N content in steel remarkably in somecases. A more preferred range of WCA is from 0.1 to 0.2 kg/(t-moltensteel).

3. Best Mode of Process of Producing Steel for Steel Pipes

The best mode of the method of the present invention is, as describedabove, the method of producing steel for a steel pipe excellent insour-resistance performance described in any one of claims 1 to 3 thatis for melting and refining extra-low-sulfur high-cleanliness steelexcellent in sour-resistance performance, wherein molten steel istreated by the steps indicated in Steps 1 to 4 below and subsequentlyadding Ca in Step 5 below. Here, the method includes the followingsteps: Step 1: CaO-type flux is added to molten steel in a ladle atatmospheric pressure; Step 2: after Step 1 above, the molten steel andthe CaO flux are stirred by injecting a stirring gas into the moltensteel in the ladle at atmospheric pressure and also an oxidizing gas issupplied to the molten steel to thereby mix the CaO-type flux withoxides generated by reaction of the oxidizing gas with the molten steel;Step 3: the supply of the above oxidizing gas is halted, anddesulfurization and the removal of inclusions are carried out byinjecting a stirring gas into the molten steel in the ladle atatmospheric pressure; Step 4: an oxidizing gas is supplied into an RHvacuum chamber to increase the molten steel temperature when the abovemolten steel in the ladle is treated using an RH degasser after Step 3above, and subsequently the supply of the oxidizing gas is halted, andthen the circulation of the molten steel within the RH degasser iscontinued to remove inclusions in the molten steel; and Step 5: metallicCa or a Ca alloy is added to the above molten steel in the ladle afterStep 4 above.

Hereinafter, a suitable aspect to carry out a melting and refiningmethod according to the present invention will be described in moredetail.

3-1. Step 1

3-1-1. Time Period for Addition, Method of Adding and Amount of Additionof CaO-Type Flux

In this Step, molten steel is tapped after the completion of converterblowing and a part or the whole of the CaO-type flux used for moltensteel desulfurization treatment is added to the upper part of the moltensteel accommodated in the ladle. As the amount of Al addition and theamount of an oxidizing gas supply are determined according to a targettemperature and a target Al content and a target S content, the amountof CaO-type flux according to them is added. The CaO-type flux in apredetermined amount may be added in a lump sum or in fractionalamounts.

Treatment becomes simple and easy in case of adding in a lump sum, whileadding in fractional amounts makes it easy to melt and form slag.However, the total addition amounts of CaO-type fluxes in Steps 1 and 2need to be grasped so that all of them be added by the completion of thesupply of an oxidizing gas in Step 2. The reason is that, in utilizinggenerated Al₂O₃ in the present invention, the reaction of the flux withthe generated Al₂O₃ does not proceed sufficiently if the CaO-type fluxwere added after the supply of the oxidizing gas, and the promotion ofslag melting and forming could possibly become insufficient. Inaddition, the reason is that since the CaO-type flux has a high meltingpoint, it is preferable to further promote the melting of the CaO-typeflux and slag formation making use of the high temperature region thatis formed by supplying an oxidizing gas in following Step 2.

Additionally, although the CaO-type flux may be added after thecompletion of supply of an oxidizing gas in order to, for example, raisethe melting point of slag in the ladle, it is an improved technology ofthe present invention, and the present invention does not exclude suchflux addition.

The CaO-type flux means a king of flux in which the CaO content is 45%or more and, for example, the flux made up of single quicklime orprincipal quicklime and a blend of Al₂O₃, MgO, etc. can be used.Moreover, a premelt synthetic slag agent with good slag formingcharacteristics like calcium aluminate may be used. The slag chemicalcomposition on molten steel should be controlled within a proper rangefrom Step 3 onwards in performing desulfurization and purification tomelt and refine an extra-low-sulfur high-cleanliness steel. For thatpurpose, the CaO-type flux is preferably added in an amount of 6 kg/t ormore, more preferably 8 kg/t or more, in terms of converted CaO, by thecompletion of supply of an oxidizing gas in Step 2.

The method of adding of the CaO-type flux can be any one of (1)injecting its powders into the molten steel via a lance, (2) sprayingits powders onto the molten steel surface, (3) placing it on moltensteel in the ladle, and (4) further adding it into the ladle at the timeof tapping molten steel from the converter, and the like. However, inthe inventive method of processing at atmospheric pressure, the methodof adding the total amount of CaO-type flux into the ladle at the timeof tapping, although facilities dedicated for such as injecting orspraying are not used, is simple and easy and suitable.

It is preferred that the chemical composition of molten steel in theladle before the addition of the CaO-type flux is set to be C, 0.03 to0.2%, Si: 0.001 to 1.0%, Mn: 0.05 to 2.5%, P: 0.003 to 0.05%, S: 11 to60 ppm, and Al: 0.005 to 2.0%, and the temperature is set to about 1580to about 1700° C. However, the adjustment of these elements of moltensteel may be carried out after the addition of CaO and before the supplyof an oxidizing gas.

3-1-2. Method of Adding and Amount of Addition, etc. for Al

By the addition of Al, a heat source for molten steel heating-up in thefollowing Steps and Al₂O₃ source are supplied. Al reduces oxygen inmolten steel and iron oxide in slag and finally becomes Al₂O₃ in theslag. Al lowers the melting point of the slag, and effectively functionsfor the desulfurization and purification of the molten steel.

The slag chemical composition on molten steel should be controlledwithin a proper range after Step 3 to achieve desulfurization andpurification to melt and refine extra-low-sulfur high-cleanliness steel.Al, totaled from Step 1 to Step 2, by the completion of supply of anoxidizing gas, is preferably added in an amount of 1.5 kg/t or more,more preferably 2 kg/t or more, in terms of metallic Al equivalent. Thisis because, if the amount of addition of Al is less than 1.5 kg/t, theamount of Al₂O₃ generated is too small, and the amount of addition ofCaO needs to be adjusted while the effect of using Al for slag controlbecomes small. In addition, the effect of sufficiently decreasing lowergrade oxides in the slag also becomes small, so that variation in theeffect becomes slightly large.

The method of adding Al, like the method of adding the CaO-type flux,can use any of (1) a method of injecting the powders into the moltensteel via a lance, (2) a method of spraying the powders onto the moltensteel surface, (3) a method of placing the powders on molten steel inthe ladle, and further (4) a method of adding Al into the ladle at thetime of tapping molten steel from the converter, and the like.Additionally, as an Al source, either pure metallic Al or an Al alloymay be used, or the residue or the like at the time of Al smelting canalso be used.

Moreover, when molten steel subjected to converter blowing is tapped toa ladle, the inflow of a converter slag to the ladle is preferablysuppressed. This is because the converter slag contains P₂O₅ and notonly causes the P content in molten steel to rise in a subsequentdesulfurization treatment step, but makes it difficult to control theslag chemical composition when the amount of inflow slag to the ladlevaries. To that end, it is preferred to decrease the outflow of a slagfrom the converter to suppress the inflow of a slag into the ladle bymeans of, for example, decreasing the formation of a converter slag,introducing a blade-shaped dart to immediately above a molten steeltapping port during converter tapping to suppress the formation ofvortexes of molten steel in the upper part of the molten steel tappingport, and further detecting the outflow of a slag from the converter byan electrical, optical or mechanical method to halt the molten steeltapping flow in accordance with the timing of the slag outflow.

Not only Step 1 but also either Step 2 or Step 3 described below is alsocarried out at atmospheric pressure. The reason is that besides the factthat strong stirring operation under reduced pressure does not need tobe performed in the present invention, facility and running costs areincreased when the processes of Steps 1 to 3 are performed under reducedpressure.

3-2. Step 2

In Step 2, the molten steel and the CaO-type flux are stirred byinjecting a stirring gas into the molten steel in the ladle atatmospheric pressure to which the CaO-type flux is added in Step 1, andalso an oxidizing gas is supplied to the molten steel to thereby mix theCaO-type flux with oxides such as Al₂O₃ generated by reaction of theoxidizing gas with the molten steel.

As described above, a part of or the whole of CaO-type flux may be addedin Step 2, or a part of or the whole of Al may be added in Step 2.However, the amount of addition of CaO and Al directly concerned in thepresent invention means the amount including not only the one put in theladle before the start of the molten steel tapping from the convertorbut also those used from the start of molten steel tapping until thecompletion of supply of an oxidizing gas in Step 2.

3-2-1. Method of Supplying Oxidizing Gas

The reason why an oxidizing gas is supplied to molten steel in Step 2 isthat the heat up of the molten steel or the suppression of a temperaturedecrease is to be promoted by making use of oxidation exothermicreaction caused by reaction of molten steel chemical elements with anoxidizing gas, and also Al₂O₃ is to be generated to control the chemicalcomposition of a slag. The above kind of gases that have capability tooxidize chemical elements in molten steel can be used as this oxidizinggas.

The methods of supplying an oxidizing gas that can be used include (1) amethod of injecting an oxidizing gas into molten steel, (2) a method ofspraying an oxidizing gas from a lance or a nozzle placed above moltensteel, and the like. Among all, the method of spraying the gas to thesurface of molten steel using a top lance is preferred, from theviewpoints of slag melting and improvements of slag formation byutilization of the controllability of a slag chemical composition and ahigh temperature region. The preferred method can directly heat theCaO-type flux to promote the formation of slag of the CaO-type flux bymaking use of the high temperature region formed by reaction of anoxidizing gas with molten steel in the ladle.

When an oxidizing gas is sprayed to molten steel from a lance or anozzle placed above the molten steel, the intensity of spraying theoxidizing gas should be secured to some extent to effectively transmitgenerated heat to slag. The height of the lance should be lowered toapproach the molten steel in order to secure this spraying intensity. Asa result, the lance life span decreases due to radiant heat receivedfrom the molten steel to increase the replacing work of the lance, sothat it is difficult to maintain high productivity. Therefore, when anoxidizing gas is sprayed to molten steel through a lance or a nozzle,the lance or the nozzle is preferably made to be a water-cooledstructure.

The height from the molten steel surface to the lance or nozzle (i.e.,the vertical distance from the molten steel surface to the lance lowerend) is preferably set in the range of about 0.5 to about 3 m. This isbecause, if the height of the lance or nozzle is less than 0.5 m, thespitting of the molten steel gets active and also the life span of thelance or nozzle could be possibly shortened, while if the height exceeds3 m and becomes large, the oxidizing gas jet scarcely reaches the moltensteel surface, whereby the oxygen efficiency in refining could bepossibly extremely lowered.

3-2-2. Amount of Supply, etc. of Oxidizing Gas

The amount of supply of an oxidizing gas in Step 2 is preferably 0.4Nm³/t or more, more preferably 1.2 Nm³/t or more, in pure oxygenequivalent. This amount of supply of oxygen is the one that is preferredto obtain a heat source for maintaining and increasing the temperatureof molten steel by oxidizing Al, and also the one that is preferred foralso promoting slag forming of a CaO source added in Step 1. Adjustingthe amount of supply of oxygen to the above amount generates an amountof Al₂O₃ suitable for slag formation and makes the controllability ofthe slag chemical composition better and further improves thedesulfurization and purification function of the molten steel.

In addition, the feed rate of an oxidizing gas is preferably made in therange of 0.075 to 0.24 Nm³/(min·t) in pure oxygen equivalent. If thefeed rate of an oxidizing gas is less than 0.075 Nm³/(min·t), thetreatment time becomes long, which could possibly lower theproductivity. On the other hand, if the feed rate exceeds 0.24Nm³/(min·t) and becomes high, even though the CaO-type flux can besufficiently heated, the feed time of an oxidizing gas becomes short andat the same time the amount of generation of Al₂O₃ per unit time isincreased too much, so that a sufficient time for homogenizing themelting of slag and the chemical composition of slag could not besecured. Moreover, the life span of a lance and a ladle refractory couldbe lowered. Additionally, the feed rate of an oxidizing gas is morepreferably set at 0.1 Nm³/(min·t) or more from the viewpoint of securingproductivity.

In Step 2, the supply of an oxidizing gas that is performed as describedabove causes Al₂O₃ to be generated and also the molten steel temperatureto increase. In addition, the slag melting and slag formation arepromoted by making use of the high temperature region present at thefiring point. Additionally, Al₂O₃ generated by reaction of an oxidizinggas with molten steel is mixed with the CaO-type flux by injecting astirring gas from a lance immersed in the molten steel to therebycontrol the chemical composition of the slag.

The oxides generated by reaction of an oxidizing gas with molten steelinclude Al₂O₃ primarily and concurrently small amounts of FeO and MnO,and even SiO₂ are also generated. Either of these oxides causes themelting point of CaO to be decreased. These oxides exhibit the functionof decreasing the melting point of slag by mixing with CaO, and thuspromote the slag formation of the CaO-type flux. Here, FeO and MnO ofthese oxides have the function of increasing the oxygen potential ofslag, and thus thermodynamically disadvantageously act on thedesulfurization of molten steel, and finally react with Al in the moltensteel due to gas stirring in the subsequent Step 3 to thereby disappear.

3-2-3. Method of Injecting Stirring Gas and Amount of Injection

The methods of stirring in Step 2 include (1) a method of introducing astirring gas into molten steel through a lance immersed in the moltensteel, (2) a method of introducing a stirring gas from a porous plugplaced on the bottom of a ladle, and the like. Amongst, it is preferredto introduce a stirring gas into molten steel through a lance immersedin the molten steel. The reason is that, for a method of introducing astirring gas from a porous plug placed on the bottom of a ladle and thelike, the introduction of gas at a sufficient flow rate is difficult andthus mixing of slag with Al₂O₃ becomes insufficient; as a result, themelting and refining of extra-low-sulfur steel may become difficult.

The flow rate of injection of a stirring gas is preferably made in therange of 0.0035 to 0.02 Nm³/(min·t). This is because, if the flow rateof injection is less than 0.0035 Nm³/(min·t), the stirring power comesup short and thus the stirring of slag and Al₂O₃ becomes insufficientand also the oxygen potential of the slag is increased, whereby adecrease in oxygen potential of the slag in Step 3 that is a subsequentStep becomes insufficient, which could possibly be disadvantageous indesulfurization. On the other hand, if the flow rate of injectionexceeds 0.02 Nm³/(min·t) and becomes large, the generation of splashbecomes extremely large, which could lower the productivity. The flowrate of injection is more preferably set to be 0.015 Nm³/(min·t) or lessin order to lower the oxygen potential of the above slag as much aspossible and to avoid a decrease in productivity.

3-3. Step 3

Step 3 involves halting the supply of an oxidizing gas by use of a toplance or the like, and also performing desulfurization and removinginclusions by continuing the stirring of molten steel and slag by meansof the injection of a stirring gas via the lance immersed in the moltensteel in the ladle or the like at atmospheric pressure.

3-3-1. Method of Injecting Stirring Gas and Amount of Injection

The injection time of the stirring gas after the halt of supply of anoxidizing gas is preferably set to be 4 minutes or more, more preferably20 minutes or less. In addition, the amount of injection of a stirringgas is preferably set in the range of 0.0035 to 0.02 Nm³/(min·t). Thereason why the continuation of stirring under the above conditions ispreferred in melting and refining extra-low-sulfur high-cleanlinesssteel will be described in the following.

In Step 2, it is considered that the feed rate of an oxidizing gas isdecreased or an oxidizing gas is supplied while injecting a large amountof a stirring gas into molten steel at atmospheric pressure in order notto increase the oxygen potential of slag at the time of supply of theoxidizing gas.

However, when the feed rate of an oxidizing gas is extremely lowered,the rate of temperature rise of molten steel is decreased, therebylowering the productivity. Additionally, when an extremely large amountof stirring gas is injected into molten steel at atmospheric pressure,the spattering/splashing of the molten iron increases, leading to a costincrease due to a decrease in iron yield and/or a decrease inproductivity attributable to the adhesion of spattered/splashed bulkmetal to peripheral equipments, or the like.

In the inventive method, with a view to preventing an increase in theoxygen potential of slag due to the feed of an oxidizing gas withoutcausing the above-mentioned problems, the stirring of molten steel andslag in the ladle is separately performed in the supply period of anoxidizing gas (Step 2) and in a subsequent period without supply of anoxidizing gas (Step 3). In other words, even after the supply of anoxidizing gas by a top lance or the like is halted, the injection of astirring gas into the molten steel is continued through a lance immersedin the molten steel in the ladle, or the like. The concentration oflower grade oxides in the slag is lowered by implementing this Step, andthe desulfurization ability of the slag can be exhibited to the maximum.In addition, under usual gas supply conditions, the ratio (t/t₀) of thestirring gas injection time t in Step 3 to the oxidizing gas supply timet₀ in Step 2 is preferably set to be 0.5 or more.

In Step 3, both desulfurization and separation of oxide-type inclusionsgenerated by supplying an oxidizing gas in Step 2 are carried out at thesame time. The gas stirring time by stirring gas injection is preferablymade to be 4 minutes or more. This is because, if the gas stirring timeis less than 4 minutes, it is difficult to sufficiently lower the oxygenpotential of slag in Step 3 that is increased by the supply of anoxidizing gas in Step 2 and also it is difficult to secure the reactiontime for improving the desulfurization efficiency and for sufficientlylowering the total oxygen content (T. [O]). The longer the gas stirringtime, the more the low sulfur treatment and purification function areimproved. However, on the other hand, the productivity decreases and themolten steel temperature also decreases, and thus the stirring time isactually preferably set to be about 20 minutes or less.

The injection of a stirring gas carried out in Step 3 is also preferablyperformed by the method of introducing a stirring gas through a lanceimmersed in molten steel. The reason is that, for example, when astirring gas is introduced from a porous plug placed on the bottom of aladle, the gas with a sufficient flow rate is difficult to be introducedinto molten steel, and therefore FeO and MnO components in slag in Step3 cannot be sufficiently reduced, which sometimes makes it difficult tomelt and refine extra-low-sulfur steel.

The inventive method includes gas stirring treatment at atmosphericpressure as part of its features. This is because it is difficult tointensively stir the slag and metal in a small amount of gas injectionlike gas stirring under reduced pressure and also to perform gasstirring under stable gas flow conditions.

The flow rate of injection of a stirring gas is preferably set to be0.0035 to 0.02 Nm³/(min·t) as described above. This is because, if theflow rate of injection is less than 0.0035 Nm³/(min·t), the stirringpower comes up short and thus the reduction of the oxygen potential ofslag in Step 3 becomes insufficient, so that further desulfurizationcould not possibly be promoted. In addition, if the flow rate ofinjection exceeds 0.02 Nm³/(min·t) and becomes large, the generation ofsplash becomes extremely active, which could lower the productivity. Theflow rate of injection is more preferably set to be 0.015 Nm³/(min·t) orless in order to lower the oxygen potential of slag as much as possibleand to avoid a decrease in productivity.

3-3-2. Slag Chemical Composition after Completion of Step 3

For the slag chemical composition after the completion of treatment byStep 3, preferably, the mass content ratio of CaO to Al₂O₃ (hereinafter,also noted as “CaO/Al₂O₃”) is set at 0.9 to 2.5, the total mass contentsof FeO and MnO in this slag (hereinafter, also noted as “FeO+MnO”) isset at 8% or less. Further, the slag chemical composition is preferablyadjusted to have CaO in the range of 45 to 60%, Al₂O₃ in the range of 33to 46%, CaO/Al₂O₃ 1.3, and (FeO+MnO) 4%. Explicitly, it is much morepreferable to have CaO in the range of 50 to 60%, Al₂O₃ in the range of33 to 40%, CaO/Al₂O₃ 1.5, and (FeO+MnO) 1%. As a result, the controlaccuracy of the inclusions chemical composition in addition to theimprovement of cleanliness is further stabilized.

3-3-3. Steel Chemical Composition and Inclusions Control, etc. afterCompletion of Step 3

As a result of completion of treatment of Step 3, extra-low-sulfurhigh-cleanliness steel as having an S content of 10 ppm or less and a T.[O] of 30 ppm or less in molten steel is produced. The temperature atthe completion of Step 3 is about 1590 to about 1665° C.

Additionally, as described above, in Steps 1 to 3, treatments arepreferably proceeded without immersing a dip tube such as a snorkel inthe molten steel in the ladle from the viewpoint of securing an amountof slag that effectively acts on desulfurization. This is because, whenthe dip tube or the like of degasser is immersed, it partitions the slagto the one inside and the other outside thereof, and while the slageffecting of the slag in the region where an oxidizing gas is suppliedis promoted, the slag effecting of the slag present in the other regionis delayed and the stirring of the slag present outside the dip tubebecomes insufficient, whereby the amount of slag that effectively actson desulfurization could be decreased.

Here, the amount of slag after the completion of Step 3 is preferablyabout 13 to about 32 kg/t. If the amount of slag is less than 13 kg/t,it is too small, so that stable desulfurization efficiency is hardlyobtainable. Moreover, if the amount of slag exceeds 32 kg/t and becomeslarge, a time period required to control the slag chemical compositionbecomes long; as a result, the treatment time may be prolonged.

Implementing the processes of Steps 1 to 3 as described above makes itpossible to achieve desulfurization and purification of steel leading upto the extra-low-sulfur region by use of the CaO-type flux and toinexpensively melt and refine extra-low-sulfur high-cleanliness steelhaving an S content of 10 ppm or less and a T. [O] of 30 ppm or less. Inaddition, even if fluorite (CaF₂) is not added to molten steel in theladle, the desulfurization and the cleaning action of steel can besecured, so that no use of fluorite is preferred. Fluorite is recentlyscarcely available due to resource depletion, and also it is becomingless often to use it in consideration of environmental problems, wherebythe inventive method that does not require the use of fluorite issuitable as a method of melting and refining environmentally-friendlysteel.

In the melting and refining method of the present invention that makesrefining reaction proceed by supplying an oxidizing gas to molten steel,the oxidation reaction of molten steel accompanies spattering of splash,smoking and dust emission, whereby it is preferred that a cover isdisposed above the ladle to prevent the escape and also they areprocessed by a dust collector. In addition, the introduction of air canbe prevented by controlling the pressure within the above cover to be apositive pressure to thereby be able to prevent the reoxidation ofmolten steel and the ingress of nitrogen. Moreover, a non-consumable toplance is generally used for the supply of an oxidizing gas and awater-cooled lance is preferably used to improve its cooling efficiency.

3-4. Step 4

Step 4 is the step for compensating temperature while maintaining thestate of the extra low S content by suppressing “resulfurization” andfor further improving cleanliness. For this, RH equipment should beused. RH treatment involves immersing two dip tubes provided on thebottom of a vacuum tank in molten steel in the ladle and circulating themolten steel in the ladle through these dip tubes and thus is capable ofseparation treatment of inclusions in a state in which the stirring ofslag is weak and the detaining of the slag is little, thereby being ableto further conduct higher purification. In addition, since the reactionrate between slag and molten steel is small, the resulfurization can besuppressed even if temperature-raising heating is applied using RHequipment.

A method of performing temperature-raising heating of molten steel thatuses RH equipment will be described. An oxidizing gas is injected intomolten steel in a vacuum tank while circulating the molten steel betweenthe vacuum tank and the ladle by use of RH equipment, or an oxidizinggas is sprayed onto molten steel in a vacuum tank via a top lanceprovided in the vacuum tank. Oxygen in this oxidizing gas reacts with Alin the molten steel to generate Al₂O₃ and at the same time generatesheat of reaction and then the molten steel temperature rises by thisheat of reaction. Additionally, the reaction of this Al with oxygengenerates Al₂O₃ inclusions, FeO and MnO. Generated Al₂O₃, FeO, and MnOmove into the slag on the surface of the molten steel in the ladle,increasing the (FeO+MnO) content in the slag and lowering thedesulfurization ability of the slag.

On this occasion, if the reaction rate of the slag and molten steelshould be fast, a resulfurization phenomenon may occur in which S in theslag moves into the molten steel; however, the reaction rate of the slagand molten steel is slow in RH treatment, and hence the resulfurizationcan be suppressed. Therefore, shifting part of the process oftemperature-raising heating to the RH treatment from the desulfurizationtreatment enables the resulfurization to be suppressed and thetemperature to be raised while maintaining the S content in the moltensteel at a very low level.

Moreover, when more advanced purification than that at the time ofcompletion of Step 3 is required, inclusions can be further removed andcleanliness can be further improved by continuing to circulate afterhalting the supply of an oxidizing gas. The RH circulation treatmenttime after the halt of supply of an oxidizing gas in Step 4 ispreferably 8 minutes or more, more preferably 10 minutes or more, stillmore preferably 15 minutes or more. This RH circulation treatment timemay be properly determined according to a required inclusions amountlevel or hydrogen content level. The T. [O] content after RH circulationtreatment is preferably 25 ppm or less, more preferably 18 ppm or less.In addition, the N content after RH treatment is preferably 50 ppm orless, more preferably 40 ppm or less. This is because, as a result, thereduction of the amount of Ca addition and the stabilization of theinclusions composition control can be implemented. Additionally, thesupply amount of an oxidizing gas may be properly determined accordingto a molten steel aimed temperature upon raising temperature.

The feed rate of an oxidizing gas in Step 4 is preferably 0.08 to 0.20Nm³/(min·t) in pure oxygen equivalent. If the feed rate of an oxidizinggas is less than 0.08 Nm³/(min·t), the treatment time of molten steel isextended; if it exceeds 0.20 Nm³/(min·t) and becomes high, the amountsof generated FeO and MnO unpreferably increase.

The oxidizing gases that can be used include single gases such as oxygengas and carbon dioxide, mixed gases of said single gases, and blendedgases the above gases and inert gases or nitrogen gas. Oxygen gas ispreferably used from the viewpoint of shortening the treatment time.

The method of supplying an oxidizing gas can be any of those such asinjecting the gas into molten steel and spraying the gas onto thesurface of molten steel in a vacuum tank through a top lance. The methodof spraying is preferred in consideration of good operability. In thiscase, the top lance nozzles may include any shapes such as a straighttype, a steeply radially expanded type and a Laval type. In addition,the lance height (i.e., the vertical distance between the lance lowerend and the surface of molten steel in the vacuum tank) is preferablyfrom 1.5 to 5.0 m. This is because, if the lance height is less than 1.5m, the lance is very likely to be damaged due to spitting of moltensteel, and if the height exceeds 5.0 m and becomes large, the oxidizinggas jet scarcely reaches the molten steel surface, lowering theheating-up efficiency.

The ambient pressure in the vacuum tank during supply of an oxidizinggas is preferably made to be 8000 to 1100 Pa. When the circulation isperformed continuously after the halt of supply of an oxidizing gas, theambient pressure is preferably 8000 Pa or less, more suitably 700 Pa orless. If the ambient pressure in the vacuum tank exceeds 8000 Pa andbecomes high, the removal of inclusions unpreferably requires long timedue to a slow circulation rate. Additionally, at 700 Pa or less, the Hconcentration and the N concentration in molten steel can be reduced atthe same time, while allowing the removal of inclusions to beeffectively carried out.

Moreover, the composition such as Si, Mn, Cr, Ni and Ti in molten steelmay be adjusted by addition of alloying elements or the like into themolten steel during or after the supply of an oxidizing gas.

3-5. Step 5

Step 5 is the step of adding metallic Ca or a Ca alloy to molten steelin the ladle after Step 4. Suitable conditions of Ca addition are asdescribed above. The timing of Ca addition may be better to be afterStep 4, and the circulation time in Step 4 is preferably 10 minutes ormore, more preferably 15 minutes or more. On the other hand, the longerthe circulation time, the more the amount of inclusions is reduced; ifthe circulation time exceeds 30 minutes and becomes long, the effectshould be saturated and at the same time the molten steel temperaturemay be excessively lowered, which is not preferable.

Here, the method of adding Ca and the addition conditions in Step 5 arethe same as the case of the method described in the best mode of theinvention pertinent to claim 3. In addition, for the purpose ofdecreasing Ca loss by Ca evaporation, though Ca is preferably added atatmospheric pressure, it may be added in the RH in the ending timeperiod of RH treatment, preferably 3 minutes before and to the end ofthe RH treatment. In this case, though the total treatment time can beshortened, the loss of Ca is increased if the vacuum treatment iscontinued for a long time after the addition of Ca in the RH. Because ofthis, Ca is preferably added 3 minutes before and to the end of the RHtreatment.

Additionally, when Ca is added in the RH, the ambient pressure in thevacuum tank is preferably from 6 kPa to 13 kPa, both inclusive. This isbecause, if the ambient pressure is less than 6 kPa, the evaporation ofCa is activated, while if the ambient pressure exceeds 13 kPa andbecomes high, the circulation rate of molten steel decreases, wherebythe melding of molten steel becomes insufficient.

Ca may be added after the treatment in Step 4, or in the ending timeperiod of the RH treatment, preferably, 3 minutes before and to the endof the RH treatment, or after the atmosphere surrounding the ladle isestablished to be atmospheric pressure conditions. Ca is preferablyadded at atmospheric pressure for the purpose of reducing the loss of Cadue to its evaporation.

Moreover, when Ca is added at atmospheric pressure, the addition of Camay be carried out after conveying the ladle from the RH equipment tothe different location, or may be done in a tundish during casting. Inaddition, the addition of Ca may be carried out in ambient atmosphere(in air), or under conditions in which the atmosphere gas is substitutedby an inert gas such as Ar gas.

Example

Melting and refining tests on steel for a steel pipe shown in thefollowing were carried out and the results were evaluated to confirm theeffect of the method of melting and refining extra-low-sulfurhigh-cleanliness steel according to the present invention.

1. Melting and Refining Test Method

A molten pig iron subjected, as required, to hot metal desulfurizationand hot metal dephosphorization treatment in advance was charged to atop and bottom blown converter of a scale of 250-ton (t). Roughdecarburization blowing was performed until the C content in the moltenpig iron became from 0.03 to 0.2%. The end-point temperature was set tobe in the range of 1630 to 1690° C. and the rough decarburized moltensteel was tapped to a ladle. At molten steel tapping, a variety ofdeoxidizing agents and alloys were added thereto to set the molten steelcomposition in the ladle to be C, 0.03 to 0.35%, Si: 0.01 to 1.0%, Mn:0.1 to 2%, P: 0.005 to 0.013%, S: 27 to 28 ppm, sol. Al: 0.005 to 0.1%,and T. [O]: 50 to 150 ppm.

1-1. Method of Testing Inventive Example

Steel for a steel pipe was manufactured according to the productionmethod described in claim 4. As Step 1, at the time of molten steeltapping at atmospheric pressure, 8 kg/t of quicklime was added in a lumpsum to molten steel in a ladle. In addition, metallic Al of 400 kg wasadded in a lump sum during this molten steel tapping.

In Step 2, an immersion lance was immersed in the molten steel in theladle, Ar gas was injected at a feed rate of 0.012 Nm³/(min·t) and alsooxygen gas was sprayed from a top lance with a water-cooled structureonto the surface of the molten steel at a feed rate of 0.15 Nm³/(min·t).At this time, the vertical distance between the lance lower end and thesurface of the molten steel was set to be 1.8 m, and the oxygen feedtime was set to be 6 minutes. In addition, a dip tube was not immersedin the molten steel, a cover was placed above the ladle, and evolvedgas, splash, dust, etc. were led to a dust collector and processed.

In Step 3, after the supply of the oxygen gas was halted, Ar gas wasinjected for 10 minutes at a feed rate of 0.012 Nm³/(min·t) for stirringpurpose. The slag chemical composition after the completion of Step 3has 0.7 to 1.2 of CaO/Al₂O₃ and a content of (FeO MnO) of 8 to 22%.

As Step 4, oxygen gas was sprayed at 1.5 Nm³/t from a top lance placedwithin a vacuum tank immediately after the start of RH treatment. Thelance nozzle used a straight type, the vertical distance between thelance lower end and the surface of molten steel in the vacuum tank wasset at 2.5 m, and the feed rate of oxygen gas was set at 0.15Nm³/(min·t). The dip tube diameter of RH equipment is 0.66 m, the flowrate of a circulating Ar gas is 2.0 Nm³/min, and the attained vacuum is140 Pa. After the halt of supply of oxygen gas, the circulationtreatment was applied for 15 minutes to complete the treatment.Additionally, the amount of slag in the melting and refining test isabout 18 kg/t. A sample was collected from molten steel during treatmentof Step 4 and the N content in the molten steel was analyzed. Moreover,an alloy and the like were optionally charged into the molten steel, andthe final component was adjusted.

As Step 5, the ladle was transferred to another treatment position otherthan where the RH equipment is located and Ca was added at atmosphericpressure according to the method described in claim 3. Ca was added by amethod of adding wires that have an embedded CaSi alloy with genuine Caof 30%. The addition rate was set at 0.05 kg/(min·t) in terms of genuineCa. The amount of Ca addition was determined using the N contentanalyzed in the RH treatment on the basis of the relation of equation(3) above.

1-2. Method of Testing Comparative Example

Molten steel was melted and refined by the method described below byperforming the treatments of Steps 1, 3 and 5 described in claim 4.

In other words, at molten steel tapping at atmospheric pressure, 8 kg/tof quicklime was added in a lump sum to molten steel in a ladle. Inaddition, metallic Al of 400 kg was added in a lump sum during thismolten steel tapping. Next, an immersion lance was immersed in moltensteel in the ladle, and the treatment in which Ar gas was injected at afeed rate of 0.012 Nm³/(min·t) was carried out for 15 minutes.Thereafter, the ladle was transported to RH equipment, and circulationtreatment was performed for 10 minutes. During the RH treatment, analloy and the like were optionally charged into the molten steel, andthe final composition was adjusted. After the RH treatment, the ladlewas transported to another treatment position other than the RHequipment, and in that treatment position, Ca was added at atmosphericpressure. Ca was added by a method of adding wires that have theembedded CaSi alloy with genuine Ca of 30%. The addition rate was set at0.05 kg/(min·t) in terms of genuine Ca.

2. Melting and Refining Test Result

The molten steel melted and refined by the method described in 1-1. and1-2. above was cast by a continuous casting machine to produce a slab.

The major composition of the molten steel was adjusted to be C, 0.04 to0.06%, Mn: 0.9 to 1.1%, Si: 0.1 to 0.3%, P: 0.0007 to 0.013%, S: 4 to 8ppm, Cr: 0.4 to 0.6%, Ni: 0.1 to 0.3%, Nb: 0.02 to 0.04%, Ti: 0.008 to0.012%, and V: 0.04 to 0.06%.

Next, the obtained slab was heated to 1050 to 1200° C. and then wasrolled to a steel plate with a thickness of 15 to 20 mm by hot rolling.This steel plate was formed to a UO line pipe by seam welding process.In addition, this pipe was adjusted to X80 grade of API Standards. Testpieces were cut out of this pipe and their HIC resistance performanceswere evaluated according to the method stipulated in NACE TM0284-2003.That is to say, 10 test pieces with a size of 10 mm in thickness, 20 mmin width and 100 mm in length were collected from each of the abovesteel plates and these were immersed in an aqueous solution (0.5% aceticacid+5% salt) for 96 hours at 25° C. saturated with hydrogen sulfide at1.013×10⁵ Pa (1 atm). The area of HIC generated in each test piece aftertesting was measured by ultrasonic flaw detection, and then the crackarea ratio (CAR) was determined by equation (4) below. Here, the area ofthe test piece in equation (4) was set to be 20 mm×100 mm.

Crack area ratio (CAR)=(total value of area of HIC generated in testpiece/tested area of test piece)×100(%)  (4)

Moreover, the composition of the non-metallic inclusions in the steelwas quantified using a scanning electron microscope.

Table 3 showed applied treatments in each Step, N contents in steel, CaOcontents in inclusions, CaS contents in inclusions, amounts of Caaddition, values of [N]/(% CaO) and [N]/WCA, conformance to equations(1) to (3), and crack area ratios.

TABLE 3 (% CaO) (% CaS) Amount in in of Ca [N] in inclusions inclusionsaddition Test steel (% by (% by WCA Classification No. Step 1 Step 2Step 3 Step 4 Step 5 (ppm) mass) mass) (kg/t) Inventive 1 ∘ ∘ ∘ ∘ ∘ 3530 8.5 0.05 Example 2 ∘ ∘ ∘ ∘ ∘ 42 45 3.2 0.05 3 ∘ ∘ ∘ ∘ ∘ 48 52 13.50.06 4 ∘ ∘ ∘ ∘ ∘ 54 30 14.2 0.07 5 ∘ ∘ ∘ ∘ ∘ 45 45 3.8 0.06 6 ∘ ∘ ∘ ∘ ∘48 68 22.5 0.22 7 ∘ ∘ ∘ ∘ ∘ 38 62 9.5 0.15 8 ∘ x ∘ x ∘ 41 70 20.5 0.15 9∘ x ∘ x ∘ 42 70 24.3 0.20 10 ∘ x ∘ x ∘ 66 34 5.7 0.10 11 ∘ x ∘ x ∘ 23 7018.3 0.11 12 ∘ x ∘ x ∘ 65 34 15.3 0.12 13 ∘ x ∘ x ∘ 39 30 8.5 0.04 14 ∘x ∘ x ∘ 44 35 11.3 0.05 15 ∘ x ∘ x ∘ 41 65 18.5 0.21 Comparative 16 ∘ x∘ x ∘ 38 18 15.3 0.04 Example 17 ∘ x ∘ x ∘ 45 21 8.5 0.05 18 ∘ x ∘ x ∘47 23 11.3 0.05 19 ∘ x ∘ x ∘ 51 25 14.3 0.05 20 ∘ x ∘ x ∘ 45 60 25.80.23 21 ∘ x ∘ x ∘ 62 61 30.5 0.35 22 ∘ x ∘ x ∘ 55 27 25.6 0.30 23 ∘ x ∘x ∘ 25 50 31.1 0.20 24 ∘ x ∘ x ∘ 18 70 28.3 0.25 Conformance ConformanceConformance Crack to to to area Test [N]/ [N]/ equation equationequation ratio Classification No. (% CaO) WCA (1) (2) (3) (%)Cleanliness Inventive 1 1.167 700 ∘ ∘ ∘ 0 1.00 Example 2 0.933 840 ∘ ∘ ∘0 0.95 3 0.923 800 ∘ ∘ ∘ 0 0.82 4 1.800 771 ∘ ∘ ∘ 0 1.08 5 1.000 750 ∘ ∘∘ 0 0.93 6 0.706 218 ∘ ∘ ∘ 0 1.01 7 0.613 253 ∘ ∘ ∘ 0 1.09 8 0.586 273 ∘∘ ∘ 0 0.95 9 0.600 140 ∘ ∘ ∘ 0 1.20 10 1.941 660 ∘ ∘ ∘ 0 1.11 11 0.329200 ∘ ∘ ∘ 0 0.98 12 1.911 542 ∘ ∘ ∘ 0 1.07 13 1.300 975 ∘ ∘ x 0 1.14 141.257 880 ∘ ∘ x 0 0.98 15 0.631 195 ∘ ∘ x 0 1.13 Comparative 16 2.111950 x ∘ x 1.0 1.75 Example 17 2.143 900 x ∘ x 1.2 1.65 18 2.043 940 x ∘x 3.5 1.88 19 2.040 1020 x ∘ x 4.5 1.44 20 0.750 196 ∘ x x 5.0 1.85 211.016 177 ∘ x x 2.3 2.10 22 2.037 183 x x x 3.8 1.95 23 0.500 125 x x x4.7 1.77 24 0.257 72 x x x 5.1 1.91

In the description of the column of classification in this Table,“Inventive Example” indicates being within the scope of the inventiondescribed in claim 1 and “Comparative Example” indicates being outsidethe scope of the invention described in claim 1. In this Table, the“mark ∘” in Steps 1 to 5 shows that the treatment of relevant Step wasperformed, while the “mark x” not. The “mark ∘” in each conformance toequations (1) to (3) indicates that the relevant equation was satisfied,while the “mark x” not. In addition, the “amount of Ca addition” is anamount of addition of genuine Ca in the form of CaSi alloy.

Additionally, the “cleanliness index” in this Table is a numerical valuenormalized by setting the number of inclusions in Test No. 1 as thecriterion (1.0). Here, the number of inclusions was determined byobserving the sample surface of 314 mm² under an optical microscope andtotaling the number of inclusions having a size of 5 μm or more.

In Test Nos. 1 to 7, steel for a steel pipe was produced by a productionmethod that satisfies any of conditions specified in claim 3 andconditions specified in claim 4. In Test Nos. 8 to 12, the melting andrefining were carried out by a melting and refining method thatsatisfies the conditions specified in claim 3, but does not satisfy theconditions specified in claim 4, i.e., by only carrying out theprocesses of Steps 1, 3 and 5.

Moreover, Test Nos. 13 to 15 are tests that steel is melted and refinedby the melting and refining method that satisfy neither conditionsspecified in claim 4, i.e., by only carrying out the processes of Steps1, 3 and 5, nor conditions specified in claim 3.

In addition, Test Nos. 1 to 15 above all are tests of Inventive Examplesthat carried out the method of producing steel for a steel pipe,satisfying requirements described in claim 1 including the relations ofequations (1) and (2).

On the other hand, Test Nos. 16 to 24 are tests of Comparative Examplesthat do not satisfy the requirements described in claim 4, i.e., onlythe processes of Steps 1, 3 and 5 being carried out, and that show steelmade without adopting the method specified in claim 3, and yet thatcannot satisfy any one of the relations of equations (1) and (2)specified in claim 1.

Test Nos. 1 to 15 that are Inventive Examples satisfying therequirements described in claim 1 turn out that good steel for a steelpipe having no HIC at all was produced. In particular, in Test Nos. 1 to7 satisfying the requirements of both claims 3 and 4, extremely goodsteel for steel pipes exhibiting particularly excellent HIC resistanceperformance and cleanliness were produced.

On the other hand, in Test Nos. 16 to 23 that are Comparative Examplesnot satisfying the requirements of claim 1, the steel thus produced ispoor in HIC resistance performance and its crack area ratio (CAR) showeda comparatively high value of 1 to 5%.

From the above results, it has been ascertained that satisfying therequirements of claim 1 greatly stabilizes the HIC resistanceperformance of high strength HIC resistant steel and makes it possibleto lead to the production of steel for steel pipes including line pipesexcellent in sour-resistance performance.

Additionally, the comparison of the results of Test Nos. 8 to 15 withthe results of Test Nos. 16 to 24 shows that steel excellent in HICresistance performance are obtained by satisfying the conditionsspecified in claim 1 even if the conditions specified in claim 3 or 4are not satisfied. On the other hand, as seen from the results of TestNos. 1 to 7 above, it has been ascertained that satisfying therequirements of both claims 3 and 4 makes it possible to stably producesteel for steel pipes exhibiting both particularly excellent HICresistance performance and extremely high cleanliness.

INDUSTRIAL APPLICABILITY

According to the method of producing steel for steel pipes of thepresent invention, high-strength HIC resistant steel for steel pipesfurther improved in sour-resistance performance can be stably andinexpensively manufactured by optimizing the addition of a CaO-typeflux, the gas stirring of molten steel and flux, the supply of anoxidizing gas, and the Ca addition into molten steel. In high-strengthHIC resistant steel for steel pipes manufactured by the inventivemethod, low sulfur, low nitrogen and high cleanliness by virtue ofinclusions control have been achieved, so that the inventive steel isoptimal as steel for steel pipes including line pipes that requiressour-resistance performance. Therefore, the present invention can bewidely applied, on the basis of excellent economical efficiency, in therefinement and steel pipe producing areas as technology that can stablysupply high-strength HIC resistant steel with high performance.

1. A method of producing steel for a steel pipe excellent insour-resistance performance, the steel comprising, in % by mass, C, 0.03to 0.4%, Mn: 0.1 to 2%, Si: 0.01 to 1%, P: 0.015% or less, S: 0.002% orless, Ti: 0.2% or less, Al: 0.005 to 0.1%, Ca: 0.0005 to 0.0035%, N,0.01% or less, and O (oxygen): 0.002% or less, the balance being Fe andimpurities, wherein the amount of Ca addition into molten steel in aladle, where the non-metallic inclusions in the steel include Ca, Al, Oand S as main components, is controlled according to the N content inthe molten steel prior to Ca addition such that the CaO content in theinclusions is in the range of 30 to 80%, the ratio of the N content inthe steel to the CaO content in the inclusion satisfies the relationexpressed by equation (1), and the CaS content in the inclusionsatisfies the relation expressed by equation (2).0.28≦[N]/(% CaO)≦2.0  (1)(% CaS)≦25%  (2) where [N] represents the mass content (ppm) of N in thesteel, (% CaO) represents the mass content (%) of CaO in the inclusions,and (% CaS) represents the mass content (%) of CaS in the inclusions. 2.The method of producing steel for a steel pipe excellent insour-resistance performance according to claim 1, the steel comprisingone or more of compositional elements selected from one or more ofgroups (a) to (c) below, in place of a part of Fe: (a) in % by mass, Cr:1% or less, Mo: 1% or less, Nb: 0.1% or less, and V: 0.3% or less; (b)in % by mass, Ni: 0.3% or less, and Cu: OA % or less; and (c) in % bymass, B: 0.002% or less.
 3. The method of producing steel for a steelpipe excellent in sour-resistance performance according to claim 1,wherein Ca is added such that in controlling the amount of Ca additioninto the molten steel in the ladle, the ratio of the N content in moltensteel to the amount of Ca addition to the molten steel satisfies therelation expressed by equation (3) below according to the N content inthe molten steel prior to the Ca addition:200≦[N]/WCA≦857  (3) where [N] represents the mass content (ppm) of N inthe molten steel prior to the Ca addition and WCA represents the amountof Ca addition (kg/t-molten steel) to the molten steel.
 4. The method ofproducing steel for a steel pipe excellent in sour-resistanceperformance according to claim 1, wherein the molten steel is treated bythe steps indicated by Steps 1 to 4 and then the Ca is added in Step 5:Step 1: CaO-type flux is added to molten steel in a ladle at atmosphericpressure; Step 2: after Step 1, the molten steel and the CaO flux arestirred by injecting a stirring gas into the molten steel in the ladleat atmospheric pressure, and also an oxidizing gas is supplied to themolten steel to thereby mix the CaO-type flux with an oxide generated bythe reaction of the oxidizing gas with the molten steel; Step 3: thesupply of the oxidizing gas is halted and desulfurization and theremoval of inclusions are carried out by injecting a stirring gas intothe molten steel in the ladle at atmospheric pressure; Step 4: anoxidizing gas is supplied into an RH vacuum chamber to increase themolten steel temperature when the molten steel in the ladle is treatedusing an RH degasser after step 3, and subsequently the supply of theoxidizing gas is halted, and then the circulation of the molten steelwithin the RH degasser is continued to remove inclusions in the moltensteel; and Step 5: metallic Ca or a Ca alloy is added to the moltensteel in the ladle after Step
 4. 5. The method of producing steel for asteel pipe excellent in sour-resistance performance according to claim2, wherein Ca is added such that in controlling the amount of Caaddition into the molten steel in the ladle, the ratio of the N contentin molten steel to the amount of Ca addition to the molten steelsatisfies the relation expressed by equation (3) below according to theN content in the molten steel prior to the Ca addition:200≦[N]/WCA≦857  (3) where [N] represents the mass content (ppm) of N inthe molten steel prior to the Ca addition and WCA represents the amountof Ca addition (kg/t-molten steel) to the molten steel.
 6. The method ofproducing steel for a steel pipe excellent in sour-resistanceperformance according to claim 2, wherein the molten steel is treated bythe steps indicated by Steps 1 to 4 and then the Ca is added in Step 5:Step 1: CaO-type flux is added to molten steel in a ladle at atmosphericpressure; Step 2: after Step 1, the molten steel and the CaO flux arestirred by injecting a stirring gas into the molten steel in the ladleat atmospheric pressure, and also an oxidizing gas is supplied to themolten steel to thereby mix the CaO-type flux with an oxide generated bythe reaction of the oxidizing gas with the molten steel; Step 3: thesupply of the oxidizing gas is halted and desulfurization and theremoval of inclusions are carried out by injecting a stirring gas intothe molten steel in the ladle at atmospheric pressure; Step 4: anoxidizing gas is supplied into an RH vacuum chamber to increase themolten steel temperature when the molten steel in the ladle is treatedusing an RH degasser after step 3, and subsequently the supply of theoxidizing gas is halted, and then the circulation of the molten steelwithin the RH degasser is continued to remove inclusions in the moltensteel; and Step 5: metallic Ca or a Ca alloy is added to the moltensteel in the ladle after Step
 4. 7. The method of producing steel for asteel pipe excellent in sour-resistance performance according to claim3, wherein the molten steel is treated by the steps indicated by Steps 1to 4 and then the Ca is added in Step 5: Step 1: CaO-type flux is addedto molten steel in a ladle at atmospheric pressure; Step 2: after Step1, the molten steel and the CaO flux are stirred by injecting a stirringgas into the molten steel in the ladle at atmospheric pressure, and alsoan oxidizing gas is supplied to the molten steel to thereby mix theCaO-type flux with an oxide generated by the reaction of the oxidizinggas with the molten steel; Step 3: the supply of the oxidizing gas ishalted and desulfurization and the removal of inclusions are carried outby injecting a stirring gas into the molten steel in the ladle atatmospheric pressure; Step 4: an oxidizing gas is supplied into an RHvacuum chamber to increase the molten steel temperature when the moltensteel in the ladle is treated using an RH degasser after step 3, andsubsequently the supply of the oxidizing gas is halted, and then thecirculation of the molten steel within the RH degasser is continued toremove inclusions in the molten steel; and Step 5: metallic Ca or a Caalloy is added to the molten steel in the ladle after Step 4.